1. Field of the Invention
The present invention relates generally to data communications, and more particularly to a circuit for matching a digital transmission on a subscriber line, where the matched transmission is used for echo-cancellation.
2. Description of the Related Art
FIG. 1 shows a communication system 10 which includes a subscriber line interface circuit 100 coupled to one end of a subscriber line 110. The subscriber line 110 is a communication medium for carrying voice and/or data signals. One example of a subscriber line 110 is a conventional telephone line comprised of a twisted-pair of copper wires. The subscriber line 110 includes a tip line 112 and a ring line 114. According to one configuration for which the present invention is suited, the subscriber line 110 is a two-way communication medium, whereby the tip and ring lines 112 and 114 together carry both signals being transmitted and signals being received. The transmission signals and receive signals overlap each other in both time and frequency domains on the communication medium. In effect, the communication medium carries a composite signal representing overlapping transmission and receive signals.
The line interface 100 includes a transmission path 122 and a receive path 124, each of which having lines corresponding to the tip line 112 and the ring line 114. The line interface 100 is coupled to the subscriber line 110, typically by a coupling transformer 115. A driver 120 drives transmission signals onto the subscriber line 110. The driver is preferably a low-impedance driver. A pair of isolation resistors RT and RL are resistively matched as close as possible to an impedance ZSL of the subscriber line at the transformer 115. The impedance ZSL is based on an impedance of the subscriber line ZLOOP as it is converted through the transformer 115, as seen through the turn ratio of the coils of the transformer 115. Secondarily, ZSL is also based on other attributes, such as capacitance, for example, of the transformer 115 and other potential circuit components of the line interface 100, which are not shown in FIG. 1.
Point A in FIG. 1 represents a point between the output of the driver 120 and the isolation resistors, RT and RL. The driver 120 has a very low output impedance, approaching zero, so that transmission signals on the output of the driver 120 are not affected by signals being received. Thus, signals on the driver 120 side of isolation resistors RT and RL are largely, if not exclusively, transmission signals. On the subscriber line side of the terminal resistors RT and RL, several attributes of the communication system 10 transform the transmission signals. The attributes include an impedance of the subscriber line 110 and an impedance of the coupling transformer 115 At that point, the transmission signals are also combined with, and affecting, the signals being received. Point B in FIG. 1 represents a point on the subscriber line interface circuit where transformed transmission signals are combined with receive signals.
As both receive and transmit signals are present on the subscriber line, and have overlapping spectral content, signals being received must be isolated from transmission signals at the receiving end, i.e. at the line interface circuit 100. However, such a procedure is very complex, due to the difficulty of determining the signal being transmitted and its effect on the signal being received. This difficulty exists because the transmission signals are transformed from a known signal at the point where they are output from the driver 120, to a transformed signal at the point where they reach a transformer coupled to the subscriber line 110, influenced by a plurality of transforming attributes. Most of the transformation is related to the attributes of the communication system 10 described above.
Signals arriving at the line interface 100 have attenuated extensively, and thus make up a smaller relative portion of the combined signal present on the subscriber line 110. Therefore, some line interfaces employ a device known as a hybrid circuit 130 to approximate the transformation of the transmission signals. The hybrid circuit 130 is configured to produce a simulated transformed transmission signal in order to remove any transformed transmission signals from the receive signals.
Operation of the line interface 100 shown in FIG. 1 occurs as follows. The hybrid circuit 130 receives a pure transmission signal from the driver, and transforms it based on approximated characteristics of the subscriber line at the line interface 100. A transformed transmission signal, representing a transmission signal that would occur at a point where it is combined with a receive signal, is passed to a subtractor 150. A composite signal, having both transmission signals and receive signals, is coupled and filtered by a filter 140, to remove aliasing or interfering frequencies. At the subtractor 150, the signal provided by the hybrid circuit 130 is subtracted from the composite signal provided by the filter 140, to theoretically yield only a receive signal. The recovered receive signal is then passed on for digital signal processing. The anti-aliasing filter 140 is normally provided separately from the hybrid circuit 130 to ensure a receive signal does not exhibit aliasing when the digitization process occurs in the DSP.
Conventional hybrid circuits, therefore, generally take a xe2x80x9cknownxe2x80x9dsignal being transmitted, and approximate how that signal will change in the presence of signal-transforming attributes or characteristics of the subscriber line. An approximated transmission signal is needed so that it may be removed from a receive signal. Accordingly, a hybrid circuit should model the transforming characteristics of the subscriber line on a transmission signal as accurately as possible. Conventional hybrid circuits are limited in how well they model those transforming characteristics of a given subscriber line.
The present invention is a circuit and method for modeling a plurality of attributes of a communication system that includes a line interface coupled to a subscriber line. The plurality of attributes collectively transform a transmission signal prior, and in some cases as it is being combined with a receive signal on the subscriber line. The attributes include impedances from various sources on the subscriber line and circuits of a line interface circuit. The present invention accurately models the plurality of attributes in order to substantially duplicate the transforming effects of the attributes on a representation of a transmission signal that has not yet been transformed, and provide a transformed transmission signal for removal from the receive signal.
In an embodiment of the invention, a circuit includes an input for providing a representation of an outbound transmission signal that is substantially free of time and frequency overlaps with inbound receive signals from the subscriber line, and wherein transmission signal bandwidth is asymmetrical to receive signal bandwidth. The circuit further includes a modeling section connected to the input, comprising resistive and a capacitive elements which are arranged for modeling a plurality of attributes of a subscriber line environment which transform transmission signals prior to overlapping with receive signals. The modeling section is further configured to substantially duplicate a transformation on the representation of the transmission signal. An output connected to the modeling section provides the transformed representation of the transmission signal.
In another embodiment of the invention, a method includes the steps of coupling an outbound transmission prior to its overlap with an inbound receive signal, to provide a representation of the transmission signal, modeling a plurality of attributes of the communication system, the attributes having a collective transforming effect on outbound transmission signals prior to overlapping with receive signals, and with the modeling, transforming the representation of the outbound transmission signal.